Conventionally, anionic surfactants, particularly, alkylsulfates and polyoxyalkylene alkyl ether sulfates are widely used as washing components for home use and industrial use since the surfactants are excellent in washing power and foaming power. As one of the anionic surfactants, a report has been made about an olefin sulfonate, particularly, an internal olefin sulfonate obtained using, as a raw material, an internal olefin, which has a double bond not at any terminal of the olefin but at an inside thereof.
Such an internal olefin sulfonate is generally obtained by causing an internal olefin to react with a gaseous-sulfur-trioxide-containing gas to sulfate the olefin, neutralizing the resultant sulfonated internal olefin, and further hydrolyzing the neutralized product. It is known that this internal olefin sulfonate is good in biodegradability and others. However, as compared with general-purpose surfactants, such as polyoxyalkylene alkyl ether sulfates, the internal olefin sulfonate has not yet been sufficient in basic performances for washing agents, typical examples of the performances being foaming performance and foam property. Thus, the internal olefin sulfonate has been desired to be further improved. In order to improve the basic performances for washing agents, it is important to decrease internal olefins and an inorganic substance which are each by-produced in a process for producing the sulfonate.
It has been understood that the internal olefins and the inorganic substance (sodium sulfate) are generated by reverse-reactions of β-sultone, which is a reaction intermediate. JP-A-2-073051 describes an internal olefin sulfonate producing method for producing an internal olefin sulfonate having a low free-oil content, a low inorganic-sulfate content and a bright color. This method includes: causing an internal olefin to react with a sulfonating agent at a specific molar ratio in a thin-film reactor while this reaction system is cooled through a cooling means having a temperature not higher than 35° C.; neutralizing the reaction product; and then hydrolyzing the product. This publication also states that at the time of producing an internal olefin sulfonate derived from an olefin having more than 14 carbon atoms, it is necessary to attain a close mixing of the reaction product with an aqueous base in the neutralization and the hydrolysis.